1. Field of the Invention
The present invention relates generally to corrosion resistant metal alloys and, more particularly, to nickel-iron-chromium alloys which are particularly useful in corrosive oil and gas well and marine environments where high strength, corrosion resistance and reasonable cost are desired attributes.
2. Description of Related Art
As the older shallow and less corrosive oil and gas wells deplete, higher strength and more corrosion resistant materials are needed to allow for deeper drilling which encounters more corrosive environments.
Oil patch applications now require alloys of increasing corrosion resistance and strength. These increasing demands arise from factors including: deep wells that involve higher temperatures and pressures; enhanced recovery methods such as steam or carbon dioxide (CO2) injection; increased tube stresses especially offshore; and corrosive well constituents including: hydrogen sulfide (H2S), CO2 and chlorides.
Materials selection is especially critical for sour gas wells—those containing H2S. Sour well environments are highly toxic and extremely corrosive to traditional carbon steel oil and gas alloys. In some sour environments, corrosion can be controlled by using inhibitors along with carbon steel tubulars. The inhibitors, however, involve continuing high cost and are often unreliable at high temperatures. Adding corrosion allowance to the tubing wall increases weight and reduces interior tube dimensions. In many cases, the preferred alternative in terms of life-cycle economy and safety is the use of a corrosion resistant alloy for tubulars and other well components. These corrosion resistant alloys eliminate inhibitors, lower weight, improve safety, eliminate or minimize workovers and reduce downtime.
Martensitic stainless steels, such as the 13% chromium alloys satisfy corrosion resistance and strength requirements in slightly corrosive oil patch applications. The 13% alloys, however, lack the moderate corrosion resistance and strength required of low-level sour gas wells. Cayard et al., in “Serviceability of 13Cr Tubulars in Oil and Gas Production Environments,” published sulfide stress corrosion data that indicate 13Cr alloys have insufficient corrosion resistance for wells that operate in the transition region between sour gas and non-sour gas environments. Further background art may be found in U.S. Pat. No. 4,358,511 to Smith, Jr. et al. and U.S. Pat. No. 5,945,067 to Hibner et al.
While the mildly corrosive wells are handled by various 13Cr steels, Ni-base alloys are needed for the more highly corrosive environments. Among the more commonly used Ni-base alloys for oil patch use are austenite high-Ni-base alloys such as, for example, alloys 718, 725, 825, 925, G-3, C-276 which provide increased resistance to corrosive sour gas environments. These aforementioned alloys, however, are either too expensive or do not possess the necessary combination of high strength and corrosion resistance.
The present invention solves the problems encountered in the prior art by providing an alloy with excellent corrosion resistance to function in sour gas environments coupled with excellent mechanical properties for service in demanding deep well oil and gas applications. In addition, the present invention provides a high strength and corrosion resistant alloy for use in oil patch applications at a reasonable cost.